Understanding of the details involved in the steps linking activation of receptors on the cell surface with the initiation and control of cell function including growth and differentiation is a fundamental question with implications in a wide variety of improtant processes including clinical situations. A key feature of many of these activation pathways is an elevation of intracellular Ca2+ resulting at least in part from an enhanced entry of Ca2+ from the extracellular medium. The steps responsible for the ectivation of this pathway are presently unknown. To date debate on such mechanisms has focussed on responses under conditions generating a sustained elevation in [Ca2+]. It is now known that under more physiological conditions [Ca2+] responses frequently involve more complex changes including oscillations waves and spatially localized increases in [Ca2+] (so-called 'trigger zones') Ca2+ entry has generally been assumed to play only a minor role in such responses but our recent data suggest this is not so. The evidence further suggests that current models explaining the control of Ca2+ entry during a sustained elevation of [Ca2+] are not adequate to account for the features we have observed for Ca2+ entry during these more complex [Ca2+] responses. Such complex [Ca2+] signals are now believed to be of critical importance in the appropriate activation of mechanisms essential for effective exocrine fluid and protein secretion in acinar cells. Our aim is to investigate the role of Ca2+ entry in [Ca2+] oscillations and [Ca2+] trigger zones and the mechanisms controlling it. Digital fluorescence imaging of cells will be employed to obtain spatial information on the relationships between [Ca2+] responses and Ca2+ entry. In addition combined photoncounting microfluorimetry patch-clampong and flash-photolysis will be used for high temporal resolution of {cA2+] responses and their relationships with membrane potential and the activation of membrane currents. Analysis of the nature and roles of Ca2+ entry in such complex responses will significantly further our understanding of signalling pathways in cells. It may also permit the development of pharmacological agents that can be used to manipulate or modify this funddamental and widely distributed process in clinically relevant ways.